tweening

An interpolation technique where an animation
program generates extra frames between the key frames that the
user has created. This gives smoother animation without the
user having to draw every frame.

A scene is described by a mathematical model - a set of two-
or three-dimensional objects whose positions in are given by
sets of coordinates. Tweening uses mathematical formulae to
generate these coordinates at a sequence of discrete times.
The simplest system would move each point at a constant rate
in a straight line between its initial and final positions,
though other kinds of path are possible. The coordinates at
each time step are used to generate (or "render") a
two-dimensional image of the scene which forms one "frame" of
the animation.

Tweening is similar to morphing except that morphing is
usually performed by interpolating between corresponding
points marked by the user on two images, rather than between
two configurations of a model.